Go Kart Champion

Great write up. Have to say it seems quite an elegant solution. Complex (only in the way it has to be activated), yes, but so much more flexible than a mechanical lsd.

The engineers now have the ability to linearly control the amount of diff slip at any time by adding a few lines of code. You could even have a button on the dash to lock it at any time. Does it react fast enough? As fast as the anti lock system. Probably can detect slip within less than half a turn of the slipping wheel, but I don't know how fast it can pressurise the plates to react and lock.

I think we may have to change the first letter in the acronym ECU, given it no longer just controls the Engine...

It still cannot react as fast a mechanical automatically torque biasing differential, nor as predictably or smoothly. It just doesn't make any sense at all. An ATB with ABS-based traction control is more elegant, more progressive, more reliable, more durable, cheaper, simpler, and accomplishes the same series of tasks.

I'm just not seeing the reason they put this money into the system on a car that has to hit that price point. For all the cheaping out they're accused of, that is a luxurious extravagance of "why-tech".

I suspect the MK8 GTI and R will cost a bit more than the current generation, the base model mk8 GOLF is listed as €25,730 in Germany with the mk8 GOLF style starting at €28,735, not far off the current prices but up a little!
The latest mk7.5 GTi pp is listed as just over €37,000, with the R listed as over €46,000,https://www.volkswagen.de/de/modelle-und-konfigurator/der-neue-golf.html

I have to say I suspect a computer controlled diff lock is actually cheaper than a mechanical one, because it moves much of the dev work to software and iterative improvement is much easier in software than in fluids/hardware. There's got to be a reason they did it this way, and other manufacturers are following suit too and I can guarantee they ain't doing it to make your life better, it's to make theirs better.

I don't know about the reaction times either; a fluid based diff lock requires a certain amount of slip to invoke the fluid's response, but conversely the computer controlled diff can be locked at the first sign of slippage and KEPT LOCKED until the algorithm determines it's safe to let go, which may well avoid repeated lock-release-slip cycles in a mechanical LSD.

But more than this I suspect the key difference is the system is able to NOT lock the diff even when there IS some slippage, because the computer knows things about the car's inputs and overall trajectory that a simplistic mechanical diff cannot know. So for example you'll get no diff lock when using extreme steering angle, whereas you would in a standard limited slip diff - along with all the accompanying steering kickback that we know from other cars.

IMHO it's a smart solution, and if done right could produce all the benefits of a traditional limited slip diff and none of the downsides.

Drag Racing Champion

I have to say I suspect a computer controlled diff lock is actually cheaper than a mechanical one,
Second hand VAQ units from scrap car are circa £800
Brand new Wavetrak diff circa £1,000

I don't know about the reaction times either; a fluid based diff lock requires a certain amount of slip to invoke the fluid's response,Its called Preload, & a proper LSD can be altered using shims, or thicker/thinner oil, but LSD like the Wavetrak use bumps/cogs to overcome the majority of this "problem"

but conversely the computer controlled diff can be locked at the first sign of slippage and KEPT LOCKED until the algorithm determines it's safe to let go, which may well avoid repeated lock-release-slip cycles in a mechanical LSD.The VAQ unit has NO preload as it called & cannot physically detect preload & has to told to active by the cars ABS sensors…& what preload it has is controlled by the cars existing XDS systems...

But more than this I suspect the key difference is the system is able to NOT lock the diff even when there IS some slippage, because the computer knows things about the car's inputs and overall trajectory that a simplistic mechanical diff cannot know. So for example you'll get no diff lock when using extreme steering angle, whereas you would in a standard limited slip diff - along with all the accompanying steering kickback that we know from other cars.

IMHO it's a smart solution, and if done right could produce all the benefits of a traditional limited slip diff and none of the downsides.The mechanical diff will always act in a predictable & controlled manner...the VAQ unit will react differently every time around the same corner at the same speed as there will be minuet variations in the conditions that it gets its signals from the sensor on all 4 wheels via the ABS systems...

Without the existing ABS & XDS systems this VAQ unit is an 11kg dead weight...even the TCR race cars which have this unit, don't have the ABS brakes so the electronics have to rigged to allow ABS sensors to be fitted so the VAQ unit can work!

Go Kart Champion

Not sure this was necessary.. but fwiw I believe I understand what you wrote and the implications of it, and I am aware that (as you say) there's options to control the behaviour of traditional LSD and that there's an upside to "predictability", but I also think there's advantages for a computer controlled variable diff lock, just because it can behave differently for more reasons than a simplistic "detected slippage" -- as you've also stated. In fact a tuning shop can actually modify the behaviour of it daily, just because it's driven by software.

It is rather foolish of VW to describe it as a "mechanical limited slip differential" though because I think calling it such undersells what it can do as part of a Mk7/8 Golf electronics package and they should have come up with a more impressive name....

There's one other possibility - I wonder if they are doing it this way because it's more efficient? I have no idea if this is the case or not, but it's one of the main reasons car manufacturers tend to use unintuitive approaches in other areas of car design. Does that apply here?

So .. why do people dislike it so much? Is it just the perceived complexity of yet another system that used to have a mechanical governor handed over to a computer algorithm instead? Is it the risk of future service costs and failure costs outside warranty? Is it the perceived unpredictability?

Drag Racing Champion

So .. why do people dislike it so much? Is it just the perceived complexity of yet another system that used to have a mechanical governor handed over to a computer algorithm instead? Is it the risk of future service costs and failure costs outside warranty? Is it the perceived unpredictability?

Go Kart Champion

It's not as good as a mechanical ATB, locker, or LSD. It must react to something, where those units are always engaged. They are a part of the driveline. They're infinitely predictable, progressive in operation, and never throw a wrench into the driving experience.

What it allows them to do is create a holistic vehicle control system that is cheaper to manufacture than a carefully tuned mechanical system, and to offer more model variants using not only the same platform, but the same engine and transmission.

Just look at the diagrams he posted. That VAQ nonsense is a "bolt on" mod from Volkswagen; not a carefully integrated mechanical system.

The Infiniti G20T I'm selling right now has a mechanical limited slip. It's just as good as the VAQ unit in capability, and better in experience and predictability.

Go Kart Champion

I just put one of those Ebay clutch stops in. It's just a trim retainer, so it took about three seconds. Combined with the throttle adjustment, that's a massive improvement in overall feel.

Of course, you can simply not depress it all the way to begin with, but it's nice just being able to step on the thing and ignore it in everyday driving.

I haven't noticed any chatter with the ASL or whatever disengaged with the rest of the systems, but obviously a clutch stop makes launching less of a hassle.

Driving and only dipping past the point of contact is one thing, but it's a rare person that will actually shift into first at a light without hitting the floor; and since that's the most annoying part of this experience, the stop makes sense on this vehicle. I just played around with it in the driveway and rather liked it on this car.

Might remove two washers. It's fine now, but this is as close as I'd want it on a track; not really for everyday driving. For ten bucks? Yeah, it's a no-brainer.

The guy even sent me messages letting me know when planned to ship it, and when it shipped. Because it's just a plastic trim retainer, there is no harm at all to any part whatsoever in installation. Just press it in. Done.

11kg .. yeah it's heavier than a standard LSD probably when you add this on as an external unit. But I think it's a clever solution, and you've said yourself it has the potential to perform in an infinitely more advanced way than a tradtional LSD, and is likely to be measurably better at doing the job it's supposed to do - i.e. of controlling the distribution of torque between the front wheels to improve traction when it's required.

Whether that's entirely justified on a car as modestly quick as a golf GTi is another matter... but it adds to the safety margins, and you know they like safety margins. I suspect the ability to operate the diff with no pre-load during normal running will also reduce wear and improve miles per gallon, so for a lot of reasons I think it's actually a really neat bit of tech. Plus, from a manufacturing POV, the engine, gearbox and diff are one standard assembly and don't have to be changed regardless of whether the final vehicle has VAQ fitted or not, they just change the casing and the drive shaft to support adding it further down the production line.

So, would I buy one?

Hell no my Corrado VR6 has no limited slip diff, traction control nor even an electric throttle body and I never missed either even when I'm spinning up both front wheels. I think that even without this particular computerised intervention most modern cars barely pass as "driver's cars".

Drag Racing Champion

Whether that's entirely justified on a car as modestly quick as a golf GTi is another matter... but it adds to the safety margins, and you know they like safety margins.

So, would I buy one?

Hell no my Corrado VR6 has no limited slip diff, traction control nor even an electric throttle body and I never missed either even when I'm spinning up both front wheels. I think that even without this particular computerised intervention most modern cars barely pass as "driver's cars".

Its a safety system that allows them to go "look we've made it so you can put your foot down"...whilst at the same time the legal dept & safety lot can also be appeased due to the way its integrated with the ESC & other electronic safety aids..

I just hate the way they have fitted it to the TCR & CS/CCS..& give the GTI a "performance pack"....biggest load of misleading BS I've even seen....but that smarketing for you...

Go Kart Champion

11kg .. yeah it's heavier than a standard LSD probably when you add this on as an external unit. But I think it's a clever solution, and you've said yourself it has the potential to perform in an infinitely more advanced way than a tradtional LSD, and is likely to be measurably better at doing the job it's supposed to do - i.e. of controlling the distribution of torque between the front wheels to improve traction when it's required.

Whether that's entirely justified on a car as modestly quick as a golf GTi is another matter... but it adds to the safety margins, and you know they like safety margins. I suspect the ability to operate the diff with no pre-load during normal running will also reduce wear and improve miles per gallon, so for a lot of reasons I think it's actually a really neat bit of tech. Plus, from a manufacturing POV, the engine, gearbox and diff are one standard assembly and don't have to be changed regardless of whether the final vehicle has VAQ fitted or not, they just change the casing and the drive shaft to support adding it further down the production line.

So, would I buy one?

Hell no my Corrado VR6 has no limited slip diff, traction control nor even an electric throttle body and I never missed either even when I'm spinning up both front wheels. I think that even without this particular computerised intervention most modern cars barely pass as "driver's cars".

I'd say a traction aid is justified, here. This is certainly a car that can benefit greatly from being able to managed induced oversteer smoothly.

This will certainly be smoother than a limited slip, but there's no way it's going to beat out a mechanical ATB combined with ABS traction control unless you actually need a full lock; which is a rare occasion. That's actually why the Wavetrac was invented; it's just an ATB that doesn't need ABS to bind a wheel in a zero traction scenario. Always wanted to try a Wavetrac next to my Quaife units, actually.

Here's the brass tacks of it all:

When you pound the floor in a vehicle that's got an ATB, you can feel what's happening. You can feel the load being shunted from corner to corner; you can feel it happening in that differential, as well. There's no delay at all; it just happens, and you're now tapped into a more "primal" vehicle, but you've got a tight hold on the reins. That split second before you break traction now becomes ten seconds of room to explore, and you can feel all of it happening.

VW's solution has to step in afterword, but before that even happens, it takes everything away from you. There is no "feel". It can't respond instantly, either, so it has to step in early.

On the Corrado for sure I can spin away and understeer through a corner if I so choose, but if you get the power just right (which you can with a N/A engine and is rather more difficult on a turbo engine), it will actually flatten the car down on its haunches and pull it round the corner quite nicely. So, personally, I don't see a significant need for an LSD on the road. On the track or on gravel/mud is a different matter obs.

Caveat obviously I've not driven one, but I'd be surprised if the VAQ can't respond pretty much as fast as the ABS and traction control can, i.e. on barely a fraction of a turn of a slipping wheel.